Pressurized-water nuclear reactors have a primary circuit, in which the cooling water of the reactor is maintained at a high pressure of the order of 155 bars by means of a pressurizer arranged on one of the branches of the primary circuit.
The pressurizer makes it possible to keep the pressure in the primary circuit between particular specific limits either by spraying, when the pressure tends to exceed the permissible upper limit, or by the electrical heating of the primary fluid, when the pressure tends to fall below the permissible lower value. These operations are conducted inside the pressurizer which comprises particularly a casing having a domed bottom, through which electrical heating rods are introduced inside the pressurizer.
Passage sleeves are welded to the bottom of the pressurizer, each at a passage orifice. The heating rods are introduced into the sleeves and welded to their ends so as to ensure the mechanical stability and sealing of the connection.
The heating rods are produced in the form of glove fingers containing the heating resistors and having an open end at the outer end of the passage sleeve, to ensure the connection and feed of the electrical resistors.
Some heating rods can become defective during operation, and it is therefore necessary to check periodically that they are in a good operating condition.
Should the presence of a defective rod be detected, its replacement is carried out so as to continue to ensure that the pressurizer operates satisfactorily.
Conventionally, the replacement of a heating rod of a pressurizer is carried out by a sequence of operations including cross-cutting of the sleeve, extraction of defective rod, the cleaning and machining of the cut end of sleeve, the installation of a replacement rod, temporary fastening of this rod to the sleeve by means of manual welding spots, and permanent fastening of the rod by means of a continuous circular weld which ensures the junction between the rod and the end of the sleeve and is usually made by automatic welding. Various checking operations, to conducted between the different operations mentioned above, are all carried out from outside the pressurizer in a zone located underneath the lower domed bottom of its casing.
The pressurizer casing of general cylindrical shape and closed by domed bottoms is arranged with its axis of symmetry in the vertical direction, and the heating rods are held in a vertical arrangement, i.e., parallel with the axis of the pressurizer casing, by means of spacer plates fixed to the inner wall of the casing.
The heating rods, which are of limited length and which are arranged in the lower part of the pressurizer casing, are generally held by means of two spacer plates, namely, a lower spacer plate located above the bottom of the pressurizer and an upper spacer plate located above the lower spacer plate.
Should a heating rod to be replaced have deformations, such as enlargements or bulges, the diameter of which is larger than the diameter of the through-holes for the heating rods in the spacer plates and larger than the bore of the sleeves, it is no longer possible to extract the rod from outside the casing of the pressurizer simply by an axial pull on the end of the heating rod.
Such bulges of the metal casing of the heating rods usually occur in the region of the spacer plates and above the passage sleeves of the bottom of the pressurizer casing.
To date, no process and device making it possible to extract deformed heating rods from the casing of a pressurizer has been known.